100% gyarados

Second best dark type attacker in the game after bite/crunch tyranitar.

And you know what, crunch actually does damage, it's not like twister which does negative damage to the enemy.

Yes, power it up!

Depends on your goals:
- do you want to be the gym leader, independent of how bad your Gyarados is: Hell yeah.
- do you want to defend a gym in a Good way: Hell no.

Bite may suck in defense, but most Gyarados we see in gym have it anyway. And Crunch is a great defensive move, hits hard and it's hard to dodge. I'd definitely max it out to put on gyms.

Excellent vs Espeon. I had 2 of them. Each one have a Dark / Dragon but not in the same pokemon so I can't 100% effectively use as attacker. For example Dragon will be against Dragonite but quitk attack is Dark so it doesn't make effective damage. I'd like to have those gyarados moves changed. Would be great.

That gyrados is very good for defending gyms and stayin high in it.
I say power it up all the way

It's probably gonna be his best moveset down the line when he mega evolves (becomes Dark/Water) so he gets STAB on dark type moves, right now, I don't like Gyarados for anything other than high gym placement (and he looks really cool), as in most match ups you are better off with any specialist than you are with Gyarados. (STAB+SE + NVE Damage > Gyarados' better stats).